Despite recent major advances in treatment, coronary artery disease (CAD) remains the leading cause of death in both the USA and worldwide. Diagnosis and treatment of CAD accounts for the expenditure of $ 140 billion per year. Although there are other modalities used to diagnose heart disease, myocardial SPECT remains the best diagnostic tool for assessing myocardial ischemia, and thus the extent of CAD and its prognosis. Myocardial SPECT scans are by far the most common procedures in US nuclear medicine departments. For these reasons, we can expect that significant improvements in the efficacy of myocardial SPECT will have major impacts on patient health and medical practice. Myocardial Single Photon Emission Computed Tomography (SPECT) scans make use of 201TlCl or 99mTc-sestamibi (or equivalent) as an imaging tracer; a gamma camera is used to take multiple projection views of the tracer in the heart as the camera is rotated about the patient. Many of the limitations of myocardial SPECT imaging originate in deficiencies of the gamma camera. Recently, a new generation of cardiac SPECT cameras have been developed that make use of pixellated CdZnTe (CZT) detectors that directly read out electrical signals from interacting gamma rays. These cameras demonstrate improved spatial resolution, detection efficiency and energy resolution over conventional gamma cameras. The initial results from this first generation of CZT cardiac SPECT cameras seem promising. However, problems with CZT may limit the possibility of another CZT camera generation with larger CZT detectors and improved performance. CZT crystals are difficult to grow; good CZT detectors are difficult to produce, the yield is low, and they are consequently very expensive. This application proposes to develop imaging detectors from thallium bromide (TlBr), a dense, high Z, wide band gap semiconductor. TlBr has several advantages over CZT including significantly lower melting point, as- grown high resistivity at room temperature, higher Z and density so less material is required to achieve a given detection efficiency and higher mobility-lifetime product of holes making efficient orthogonal strip detectors feasible. RMD proposes to demonstrate detector modules for use in cardiac SPECT imaging that can be used in existing cardiac semiconductor detector cameras, and that should improve their performance. These detector modules will use a different readout technique, orthogonal strip detection, that reduces the number of electronic channels required to read out N2 pixels from N2 for a conventional pixellated detector to 2N. RMD's goal is to develop TlBr as a more efficient, less costly alternative to CZT for cardiac SPECT imaging.